Why is it decreasing? "That's the $100,000 question," said Bill Albert, chief program officer for the National Campaign to Prevent Teen and Unplanned Pregnancy.

Some experts say an emphasis on abstinence may have played a role. Some say concern about sexually spread diseases may have been a factor - perhaps instilled by parents who watched the AIDS crisis unfold. Still others suggest this is a generation of kids who are less inclined to experiment with drugs and sex than their predecessors.

The study, released Thursday, is based on interviews of about 5,300 young people, ages 15 to 24. It shows the proportion in that age group who said they had had some kind of sexual contact dropped in the past decade from 78 percent to about 72 percent.

There are other surveys of sexual behavior, but this is considered the largest and most reliable. "It's the gold standard," Albert said.

Health scientist Anjani Chandra of the Centers for Disease Control and Prevention described the decline in sex as small but significant. She declined to speculate on the reasons. It's difficult to look for a trend earlier than 2002 because previous surveys did not gather as much detail about various types of sex, she added.

However, data over the years on vaginal intercourse among never-married adolescents shows a steady decline since 1988. That seems to be in sync with other CDC studies showing an overall drop in teen pregnancy.

That the trend began in the late 1980s seems to undermine the idea that abstinence-only sex education - heavily emphasized during the 2001-09 presidency of George W. Bush - is the explanation, Albert said.

But it is possible those messages contributed, he added.

The leading influence on sexual activity among young adults is what parents teach and what peers are doing, experts said. And for whatever reason, smaller proportions are "doing it" than in the past.

Still, the CDC report may be surprising to some parents who see skin and lust in the media and worry that sex is rampant.